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Abstract. We report on ASCA and BeppoSAX X-ray broad band observations of the galactic low-luminosity X-ray source 4U 1700+24 performed on 1995 and 1998, respectively, and on (quasi-)simultaneous ground observations of its optical counterpart, V934 Her, from the Loiano 1.5-meter telescope. In order to better understand the nature of the source we also analyze public archival ROSAT and RXTE data as well as the RXTE ASM light curve of 4U 1700+24; we also re-analyze a 1985 EXOSAT pointing. The optical spectra are typical of a M2 III star; this allows us to determine a revised distance to the object of ∼400 pc. While these spectra do not show either any spectral change between the two epochs or any peculiar feature apart from those observed in normal red giants, the spectroscopic measurements carried out in X-rays reveal a complex and long-term variable spectrum, with a clear soft excess. The X-ray spectral properties of the source are best described by a thermal Comptonization spectrum plus a soft energy (<1 keV) excess, which can be modeled in the form of a blackbody emission with kTBB ∼ 1 keV; the latter component is not detected at the lowest source flux levels. The ratio between the two components varies substantially with the source flux. The X-ray emission from the object appears to become harder as its luminosity increases: indeed, the RXTE data acquired during an outburst occurred in October-November 1997 display a hard tail, detected up to 100 keV and modeled with a comptonizing cloud which is hotter and less opaque than that seen in the low intensity state. Apart from erratic shot-noise variability on timescales of tens to thousands of seconds, no significant properties (such as pulsations or QPOs) are found from the timing analysis of the X-ray light curves extracted from the observations presented here. With the new distance determination, the 2-10 keV X-ray luminosity range spanned in the considered observations lies between ∼2×10 32 and ∼1×10 34 erg s −1 . All this information, combined with the findings by other authors, allows us to suggest that the scenario which best describes the object consists of a wide binary system in which a neutron star accretes matter from the wind of a M-type giant star. Implications of such a model are discussed.

We present optical photometry and spectroscopy of the X‐ray transient XTE J1859+226, obtained during outburst and its subsequent decay to quiescence. Both the X‐ray and optical properties are very similar to those of well‐studied black hole soft X‐ray transients. We have detected three mini‐outbursts, when XTE J1859+226 was approaching quiescence, as has been previously detected in the soft X‐ray transients GRO J0422+32 and GRS 1009–45. By 2000 August 24 the system had reached quiescence with R= 22.48 ± 0.07. The estimated distance to the source is ∼11 kpc. Photometry taken during quiescence shows a sinusoidal modulation with a peak to peak amplitude of about 0.4 mag. A period analysis suggests that periods from 0.28 to 0.47 d are equally possible at the 68 per cent confidence level. The amplitude of the quiescent light curve and the relatively low ratio of X‐ray to optical flux indicate that the binary inclination should be high. The measured colours during the outburst allow us to obtain the basic properties of the disc, which agrees well with irradiated disc model predictions.

We present optical and infrared monitoring of the 2005 outburst of the halo black hole X-ray transient XTE J1118+480. We measured a total outburst amplitude of $5:7 AE 0:1 mag in the R band and $5 mag in the infrared J, H, and K s bands. The hardness ratio HR2 (5 12 keV : 3 5 keV ) from the RXTE ASM data is 1:53 AE 0:02 at the peak of the outburst, indicating a hard spectrum. Both the shape of the light curve and the ratio L X (1 10 keV)/L opt resemble the minioutbursts observed in GRO J0422+32 and XTE J1859+226. During early decline, we find a 0.02 mag amplitude variation consistent with a superhump modulation, like the one observed during the 2000 outburst. Similarly, XTE J1118+480 displayed a double-humped ellipsoidal modulation distorted by a superhump wave when settled into a near-quiescence level, suggesting that the disk expanded to the 3:1 resonance radius after outburst, where it remained until early quiescence. The system reached quiescence at R ¼ 19:02 AE 0:03, about 3 months after the onset of the outburst. The optical rise preceded the X-ray rise by at most 4 days. The spectral energy distributions (SEDs) at the different epochs during outburst are all quasi-power laws with F / increasing toward the blue. At the peak of the outburst, we derived ¼ 0:49 AE 0:04 for the optical data alone and ¼ 0:1 AE 0:1 when fitting solely the infrared. This difference between the optical and the infrared SEDs suggests that the infrared is dominated by a different component (a jet?), whereas the optical is presumably showing the disk evolution.